SUMMARY: Root nodules formed by a riboflavin-requiring auxotroph of (TI/D-his-15) on red clover ( L.) seedlings grown with or without added riboflavin, showed changes in structure or acetylene-reducing activity when compared with effective nodules produced by the parent strain (TI). Additional riboflavin was essential for the conversion of vegetative bacteria of the mutant strain into functional, N-fixing bacteroids within the nodules. If riboflavin was not added to the plants, a large proportion of the rhizobia in the host cells failed to develop, and the nodules were almost ineffective, reducing acetylene at a very low rate. When riboflavin was added to the plant growth medium, even as late as 8 days after inoculation, most mutant rhizobia survived and continued to develop into normal bacteroids. This requirement for riboflavin was most critical 1 to 3 days after the appearance of the nodules (about 7 to 9 days after inoculation). If the auxotroph was sustained through this period with added riboflavin, the vitamin could then be removed from the plant substrate and the nodules continued to develop effectively. Impairment in the release of rhizobia from infection threads was observed in older nodules formed by TI/D-his-15 in the absence of riboflavin, the rhizobia becoming enveloped in large masses of polysaccharide-like material within the host cells.


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